1. Field of the Invention
This invention relates to refractory metal carbide grade powders. Such powders contain a refractory metal carbide, a matrix metal and a pressing aid.
It also relates to a process for producing cemented carbides from such grade powders.
2. Prior Art
Grade powders are defined herein as an intimate mixture of refractory metal carbides powder plus a metallic cementing phase or matrix. Generally the grade powders include a binder which also serves as a pressing lubricant. The most common example of a grade powder is a mixture of tungsten carbide, cobalt, and paraffin wax. The carbide powder can consist of other carbides or mixtures thereof and are generally the refractory carbides as used herein include carbides of metals from the groups IV, V, and VI metals that have a melting point above about 1895.degree. C. Cobalt is the most common matrix, at least for WC, but, nickel, iron, and molybdenum either singly, in combination, or in combination with cobalt are sometimes used particularly when refractory metals other than tungsten are used. For example, the matrix phase for TiC is either nickel or a nickel-molybdenum alloy, thus as used herein the matrix metal is selected from the iron group of metals and alloys of the iron group of metals.
The most common practice for producing carbide grade powders involves a sequence of operations consisting of ball milling, drying and granulation. While this seems relatively straightforward, there are many intermediate processes and handling steps that complicate the operation. Typically, as an example, powders of WC and cobalt are weighed in the appropriate proportions and charged into a ball mill. To prevent oxidation of the powders, milling is always done in the presence of a milling fluid. Organic fluids such as hexane, heptane, primary alcohol, acetone, and the like are used. Depending on the particular grade of powder and desired powder characteristics milling times are from many hours to several days. After milling the fluid must be removed such that a dried powder is obtained. Drying generally involves some type of distillation process so that the fluid can be recovered and reused. A typical process would be to discharge the slurry into another vessel and then with the combination of heat and vacuum remove the fluid.
More recently, a process involving close-cycle spray drying has been used to remove and recover the milling fluid.
If the spray drying process is not used, several additional steps are required after conventional drying of the powders. Typically a wax, and most commonly paraffin wax, is added to the ball mill. If wax is not added to the mill, it must be incorporated into the dried powder. This step is called waxing and is done in a variety of ways.
The dried grade powders containing wax are generally fine and fluffy and have very poor flow characteristics. It is important that the powders have good flow to facililtate transfer from a powder hopper to the die cavity during pressing. Therefore, these fine, fluffy powders are converted by an operation called granulation to a flowable powder. One common method is to press the fine powders at low pressures into a loose compact or slug. This slug is then forced through a screen. The screened product is in the form of small, irregular shaped granules which will conveniently flow into compacting dies in a more controlled manner. If the spray drying process is used a free flowing powder is obtained directly as this is one of the purposes of spray drying. That is, in addition to drying a free flowing spherical powder is obtained.
Over the years, the following process has evolved as the most used method for preparing carbide grade powders. It involves the following steps, ball milling with alcohol or acetone, tungsten carbide, cobalt and paraffin wax and drying in a close-cycle spray dry system.
While this process is a considerable improvement from the previous practice it still has disadvantages compred to the process that will be described in this invention. Some of the disadvantages are the lengthy ball milling cycle. If this type of milling is used, a flammable solvent, the use of paraffin wax and an expensive drying system.
Additionally, the products produced from ball milling contain a relatively high level of sub-micron refractory metal carbide particles. During the subsequent sintering process, the fine particles preferentially and quickly dissolve in the binder and upon cooling become deposited upon the surfaces of the undissolved carbide. This procedure is known as grain growth and lowers the strength of the subsequently produced cemented carbide articles. Various techniques for reducing the amount and level of grain growth have been developed. The most commonly used technique for reducing grain growth is to use an additive which interferes with the grain growth mechanism. Another method not now widely used is a hot pressing technique. The hot pressing technique is described in U.S. Pat. No. 3,451,791.
Attritor milling has been used recently for particle size reduction in place of ball milling because a given particle size reduction can be achieved in a shorter period of time than ball milling. In the production of grade powders of the subsequent production of cemented refractory metal carbides the purpose of ball milling is not to reduce the size of particles but rather to uniformly distribute the binder phase throughout the larger amount of the carbide phase.
The organic fluids previously used as milling aids, such as hexane, heptane, the primary alcohols, acetone and the like, are all flammable materials thus extreme safety precautions must be taken to prevent air leakage into the system used to remove the milling aid. The vapors from these milling aids also are toxic to the worker. Hence, additionally precautions in handling are required.
It is believed, therefore, a process that can be conducted in an open system without fire and health hazards and produces a carbide grade powder having improved properties and characteristics would be an advancement in the art. It is also believed that a carbide grade powder that exhibits a marked decrease in grain growth during sintering when processed by normal sintering techniques and does not contain a grain growth inhibitor is an advancement in the art.